Congenital heart disease patients with functional single ventricle (FSV) anatomy usually require a series of surgeries during infancy and childhood in order to provide them with a chance to survive into adulthood. The second stage surgery, termed the hemi-Fontan procedure (HFP) re-routes systemic venous blood from the upper body and head into the pulmonary circulation without passing through an intervening ventricular chamber. Because blood flow into the lungs is largely a passive process, any anatomic obstruction within the pulmonary vasculature, elevated pulmonary vascular resistance, or significant ventricular dysfunction can result in patients having elevated hemi-Fontan pressures. Problems that develop within the hemi-Fontan pathway early in the staged palliation process can result in an increased risk for complications during the Fontan operation and throughout a Fontan patient's life. Therefore, maintaining optimal hemi-Fontan anatomy and physiology is important for long term success and survival for FSV patients. Currently, it is not standard practice at most institutions to routinely measure hemi-Fontan pathway pressures in the immediate post- operative period or in the ambulatory setting while awaiting the Fontan. Post-operative monitoring of the hemi- Fontan pressures requires an indwelling catheter in an upper body or neck vein. This has the potential to lead to complete thrombosis of the systemic vein, which is a devastating complication for the hemi-Fontan patient. Furthermore, serial outpatient cardiac catheterizations to measure hemi-Fontan pressures place infants at risk for procedural complications. Therefore, to provide clinicians in the ICU and in the ambulatory setting with the means to measure hemi-Fontan pressures, investigators are developing a miniaturized pressure sensor that can be implanted directly into the hemi-Fontan pathway at the time of the second stage operation. The proposed implantable pressure monitor will provide readily available hemodynamic assessment of the hemi- Fontan pathway during the immediate post-operative period without the risk of systemic venous thrombosis. Interval testing of the hemi-Fontan pathway pressure in the ambulatory setting will allow clinicians to observe for rising pressure trends, which may provide early indication of a problem that is developing and allow for earlier intervention before irreversible damage has occurred. This will be accomplished without the risk associated with cardiac catheterization. During this study investigators will complete the development of a miniature implantable wireless pressure sensor. Final design freeze will be achieved, and design control requirements, including risk analysis, biocompatibility testing, sterility, and manufacturing processes, will be completed to satisfy FDA requirements. Preclinical animal studies will be carried out, including a chronic GLP animal study, to determine the device's in vivo biocompatibility, functionality, and safety profile. Finally, this study will support investigators during the application process to obtain Investigational Device Exemption from the FDA for the initial feasibility studies in infants with these complex forms of congenital heart disease.